German Utility Model DE 92 17 359 U1 contains a circuit layout for the functional testing of a multistage transistor circuit as used in the ignition circuits of vehicle occupant protection systems. Their use in airbag trigger circuits is explicitly mentioned.
In addition, there are a large number of vehicle occupant protection systems available in the market where a central processing unit, or possibly even several output stages distributed within the vehicle, each trigger several ignition circuits for the ignitors of vehicle occupant protection systems, in particular pyrotechnic ignitors of airbag or belt tensioning gas generators. The output stages are usually integrated as Application Specific Integrated Circuits (ASICs) and feature the corresponding switching means for the ignition circuits to be controlled. Via connection lines, the proper ignitors will then be connected. DE 92 17 359 U1 contains a test circuit which, in the ignition circuit, will test the ignition circuit, in particular the switching means within such a circuit, by means of test currents which are very low when compared with actual trigger currents. The test is carried out by comparing any occurring changes in voltage potential with threshold values.
However, short-circuiting of components in or across parallel ignition circuits will so far not be detected. This type of short-circuiting may occur either within the ASIC--caused by damaged insulation areas between the various individual switching means--or by insulation defects in the connection lines from the functional unit, e.g. ASIC, to the ignitors.